The invention relates to an acousto-optic modulator for modulating a beam of optical radiation by interaction with acoustic waves in an acoustic medium in accordance with the Bragg relationship. The modulator comprises a block of material transparent to the optical radiation to be modulated and has respective opposite side faces of optical quality to provide input and output surfaces for a beam of said optical radiation. An end face is provided with electroacoustic transducer means for directing a beams of acoustic waves into said block to set up an interaction region for said beam of optical radiation between said input and output surfaces.
The operation of a modulator of the kind specified is discussed, for example, by E. I. Gordon in Proc. IEEE Vol. 54, October 1966, pages 1391-1401. FIG. 1 of the accompanying drawings is a diagram illustrating the principle of operation of such a modulator. A planar electroacoustic transducer 2, in the form of a piezoelectric wafer 3, formed for example from a monocrystal of lithium niobate, with upper and lower metallised electrodes 4 and 5, is mounted on one end face 6 of a block 1 of optically transparent material formed for example from a monocrystal of germanium. The transducer 2 is energised at a suitable high frequency, for example several MHz, causing a corresponding regular succession of parallel acoustic wavefronts, indicated by parallel lines 7, to propagate in the block as, for example, a longitudinal wave disturbance with the velocity .nu.L of a longitudinal acoustic acoustic wave in the direction indicated by the arrows 8. The associated local stress variations in the medium of the block will result in corresponding local variations in refractive index thus forming a corresponding diffraction structure which will propagate along the acoustic wave propagation path 9 in the direction 8.
A beam 10 of optical radiation to be modulated, in the present case coherent radiation generated by a laser (not shown), is directed via a lens 11 and an optical side face 12 of the block 1, across the path of the propagating acoustic wave 7 in an interaction region 13 at the Bragg angle .theta..sub.B with respect to the propagating wave structure 7, causing a diffracted beam 14 to be generated which is inclined at twice the Bragg angle .theta..sub.B to the direction of the input beam 10 in the interaction region 13. The amplitude of the diffracted beam 14 will depend on the amplitude of the acoustic wave 7, and therefore is used to form the modulated beam after passing out of the block 1 via the opposite optical side face 15. It should be noted herein that such a modulator can function equally well when non-coherent optical radiation is employed provided that the Bragg diffraction conditions are satisfied.
A difficulty with this form of modulator is that when the acoustic wave 7 reaches the far end face 16 of the block it will tend to be reflected, and some of the acoustic energy may then follow a retroreflective path back toward the transducer 2, as indicated by the arrows 17. As this reflected wave passes in the reverse direction through the interaction region 13 crossed by the beam of optical radiation 10, it may generate a weak diffracted beam but the direction of motion of the corresponding acoustic diffraction structure will be reversed relative to the optical beam and the original Bragg angle relationship will not be properly met. However, the reflected wave will continue to propagate until it reaches the transducer face 6 where some of the acoustic energy will be reflected as indicated by the arrows 18 so as to travel back in the initial propagation direction for which the Bragg relationship will be correct, and as it passes again through the interaction region 13, a corresponding delayed modulation signal will be imposed on the modulated beam 14, whose amplitude will depend on the amplitude of the reflected acoustic wave. The presence of this delayed signal whose delay will be that of the round trip of the acoustic wave via the various points of reflection, is undesirable and will adversely affect the performance of the modulator especially for data transmission and ranging.
In the paper referred to above, a modulator is illustrated in which the transverse far end wall of the block has a layer of acoustic absorber to reduce reflection, and this is also indicated in FIG. 1 by the reference 19. Examples of a suitable acoustic absorbing material in the case of a germanium block, are indium and lead although neither have the same acoustic impedance as germanium and the resulting impedance mismatch all generate a significant reflected signal which will be greater in the case of indium.
In order to reduce the direct mirror reflection from the far end face it has been proposed to incline the end face with respect to the acoustic wavefront so that the acoustic wave is reflected towards a non-optical side face of the block to which an acoustic absorbing layer, e.g. indium, has also been applied. In designing this wedge form of termination it was usual to avoid an inclination or wedge angle of 45 degrees for which it was though that the reflected incident acoustic wave would be directed perpendicularly at the side face thus providing an ideal retroreflective condition for generating an undesired return reflection. In practice, therefore, a wedge angle of about 30 degrees was employed so that acoustic energy which was not absorbed by the absorption layer on the inclined face, would undergo multiple absorptive reflections at the side faces of the block and thus be dissipated.
While some improvement has been achieved by this arrangement it has been found that the residual unwanted delayed modulation signal cannot be reduced to the extent required for some applications.